Abstract
The bottom water in the transition zone of the North Sea and Baltic Sea suffers from seasonal hypoxia, usually during late summer and autumn. Hypoxia is a natural phenomenon in this region because of the strong vertical stratification which prevents the bottom water to be ventilated by atmospheric oxygen. Though hypoxia occurs naturally, the frequency and the duration of these events have increased considerably during the second half of the last century in response to a long-term eutrophication trend. Several legislative actions have been implemented to reduce the land based nutrient load, but these efforts have not yet been accompanied with improved oxygen conditions. However, both anthropogenic and natural processes influence the oxygen concentrations, and it is crucial to understand the dynamics and interplay of these processes to effectively improve the water quality. This thesis provides new information about the seasonal to decadal oxygen variations in the bottom water of the Kattegat, the Danish Straits, the Sound and the Western Baltic Sea and investigates the relative importance of physical and biogeochemical processes, climate change and nutrient load reductions on the oxygen variability.
A quantitative assessment proved that a simplified oxygen model in combination with a three dimensional ocean model was able to simulate the hypoxic events and realistically reproduce interannual and seasonal oxygen variations. The good agreement between model and observations indicated that the short-term oxygen variations are to a large extent regulated by physical processes, especially wind mixing and advection. This was also documented by the relatively small contribution to the oxygen seasonality from other processes. When looking at the transition
zone as a whole, the seasonality of respiration and oxygen saturation only accounted for 13% and 27% of the seasonal oxygen variation. Furthermore, the extreme hypoxic event in 2002 could to a large extent be explained by the low autumn wind and stagnant bottom water.
On decadal timescales, model studies suggested that the oxygen consumption have on average increased by 1.32 g C m−2 yr−2 from 1975 to around 1990 but have decreased by 1.12 g C m−2 yr−2 during the 1990s and 2000s. The trends in oxygen consumption were believed to reflect the varying eutrophication level in this region. This was supported by the significant correlation between the estimated oxygen consumption trends and the concentration of dissolved inorganic nitrogen. This study highlights the necessity to consider the combined effect of climate change and eutrophication on the oxygen conditions when assessing the nutrient reduction efforts. Climatic changes have lowered the oxygen concentrations by 15-30 μmol
O2 l−1 yr−1 while the reduced nutrient concentration has increased the oxygen concentration with approximately the same amount. Thus, the positive effect on oxygen conditions from reduced nutrient concentrations have been counteracted by increased water temperatures. This has made it difficult to observe any positive effect on the oxygen conditions from the improved eutrophication status.
A quantitative assessment proved that a simplified oxygen model in combination with a three dimensional ocean model was able to simulate the hypoxic events and realistically reproduce interannual and seasonal oxygen variations. The good agreement between model and observations indicated that the short-term oxygen variations are to a large extent regulated by physical processes, especially wind mixing and advection. This was also documented by the relatively small contribution to the oxygen seasonality from other processes. When looking at the transition
zone as a whole, the seasonality of respiration and oxygen saturation only accounted for 13% and 27% of the seasonal oxygen variation. Furthermore, the extreme hypoxic event in 2002 could to a large extent be explained by the low autumn wind and stagnant bottom water.
On decadal timescales, model studies suggested that the oxygen consumption have on average increased by 1.32 g C m−2 yr−2 from 1975 to around 1990 but have decreased by 1.12 g C m−2 yr−2 during the 1990s and 2000s. The trends in oxygen consumption were believed to reflect the varying eutrophication level in this region. This was supported by the significant correlation between the estimated oxygen consumption trends and the concentration of dissolved inorganic nitrogen. This study highlights the necessity to consider the combined effect of climate change and eutrophication on the oxygen conditions when assessing the nutrient reduction efforts. Climatic changes have lowered the oxygen concentrations by 15-30 μmol
O2 l−1 yr−1 while the reduced nutrient concentration has increased the oxygen concentration with approximately the same amount. Thus, the positive effect on oxygen conditions from reduced nutrient concentrations have been counteracted by increased water temperatures. This has made it difficult to observe any positive effect on the oxygen conditions from the improved eutrophication status.
| Original language | English |
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| Publisher | The Niels Bohr Institute, Faculty of Science, University of Copenhagen |
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| Number of pages | 208 |
| Publication status | Published - 2012 |